In the field of the invention, the term “control system” is used to designate a system that serves in particular to control the operating speed of the turbomachine. By way of example, in an airplane turbojet, the system controls the flow rate of air passing through the turbojet, the flow rate of fuel into the combustion chamber, etc.
A control system generally comprises a plurality of actuators. By way of example, these actuators are air discharge valves or valves serving to adapt the geometry of the turbojet compressor.
More precisely, the invention relates to a turbomachine control system comprising an auxiliary hydraulic circuit with at least one hydraulic actuator and at least one servo-valve, the actuator being fed with liquid via the servo-valve. The hydraulic circuit of the control system is said to be “auxiliary” in order to distinguish it from the main hydraulic circuit of the turbomachine, to which it is connected.
It should be observed that hydraulic actuators are preferred over electromechanical actuators since they are more reliable and they are better adapted to the high-temperature conditions in the environment of a turbomachine. Furthermore, the use of electromechanical actuators generally constitutes a solution that is more expensive.
A known example of a control system of the above-specified type is shown in FIG. 1. The control system is fitted to airplane turbojets having a main fuel circuit 12 comprising: a low pressure pump 16 connected to a fuel tank 14; a heat exchanger 17; a high pressure pump 18; and a hydromechanical unit (HMU) 19. The low pressure and high pressure pumps 16 and 18 are mechanically driven by an accessory gearbox 22 of the turbojet. The high pressure pump 18 feeds the fuel injectors of the combustion chamber 20 of the turbojet via the HMU 19. This HMU 19 serves in particular to measure out the fuel needed by the combustion chamber 20, and to return excess fuel to the main circuit 12 upstream from the heat exchanger 17 via a recirculation loop 21.
The control system comprises an auxiliary hydraulic circuit 10 with a plurality of actuators referenced A1 to AN, where N is an integer greater than or equal to 1. There are only two actuators A1 and A2 in the example shown. Each actuator A1, A2 is fed via a servo-valve S1, S2. The auxiliary circuit 10 and its actuators A1, A2 are powered by the high pressure pump 18.
The drawback of such an installation lies in the fact that the high pressure pump 18 is driven by the accessory gearbox 22 of the turbojet, such that the hydraulic energy generated by the high pressure pump 18 depends on the speed of rotation of the drive shaft of the turbojet. Unfortunately, it is sometimes necessary to deliver a large amount of hydraulic energy to the actuators even at low speeds of rotation of the drive shaft of the turbojet. This applies in particular when relighting the turbojet in flight. An obvious modification to the system for this purpose would be to dimension the high pressure pump 18 so as to be capable of satisfying engine requirements under relighting conditions. Such a solution would be nevertheless unsatisfactory since the cylinder capacity of the high pressure pump 18 would then be much greater than required by the engine in its normal operating range. Furthermore, such overdimensioning of the high pressure pump 18 would have the drawback of increasing its weight.
Another drawback stems from the fact that the high pressure pump 18 is heavily loaded and therefore heats up more than a standard high pressure pump (i.e. a pump dedicated solely to injecting fuel into the combustion chamber 20), and the fuel passing through the pump 18 is likewise heated. Unfortunately, some of this heated fuel is reinjected into the main circuit 12 upstream from the heat exchanger 17, and is used as a cold source in the heat exchanger 17 (generally for the purpose of cooling the lubricating oil of the turbomachine). This leads to the heat exchanger 17 cooling poorly.